Providing marine vessels with at least three propulsion units in the stern is known from prior art. Examples of such solutions can be found e.g. in WO 03/066428 A1, WO 03/099651 A1, WO 2006/048460 A1 and WO 2009/007497 A2. These known solutions are mainly concerned with optimizing the loading capacity of the vessels, maintaining a balanced efficiency for cruise speeds and maneuvering, and allowing operation in ice infested waters. In all these, and other corresponding solutions, the propulsion units are located under the bottom of the hull of the vessel, closer or farther away from the stern of the vessel. Consequently, these solutions pose limits on the propellers used as well as generate hull vibrations leading to undesired hull fatigue and noise.
There are also known solutions, e.g. as disclosed in WO 2009/126090 A1, WO 2009/126096 A1 and EP 2 993 122 A1, where attempts have been made to avoid the above indicated problems. The known solution of '090 discloses a marine vessel with a single or twin propulsion unit. In case of the single propulsion unit, the solution includes a fixed shaft propeller with a rudder or a rotatable thruster including a rudder part. The twin propulsion unit includes two rotatable thrusters with respective rudder parts. The known solution of '096 presents a marine vessel with a single or twin propulsion unit including a very complex modular rotatable thruster unit installed in a tiltable container arrangement at the stern of the ship with a propeller extending aft of the stern. Publication '122 shows a vessel including a single azimuthing propulsion unit with a propeller that may extend aft of the stern de-pending on the operating mode. This may solve the vibration problems in some operational modes of the known vessels, but the known solutions strongly impede propulsion efficiency combined with adequate steering capability.